SILOptimizer: remove unused variables and fields (#63018)

Exact warning text:
```
swift/lib/SILOptimizer/Mandatory/DefiniteInitialization.cpp:3239:12: warning: variable 'NumMemoryElements' set but not used [-Wunused-but-set-variable]
  unsigned NumMemoryElements = TheMemory.getNumElements();
           ^
1 warning generated.
```

Author: MaxDesiatov

lib/SILOptimizer/Mandatory/DefiniteInitialization.cpp

@@ -3236,15 +3236,9 @@ handleConditionalDestroys(SILValue ControlVariableAddr) {
   SILBuilderWithScope B(TheMemory.getUninitializedValue());
   Identifier ShiftRightFn, TruncateFn, CmpEqFn;
 
-  unsigned NumMemoryElements = TheMemory.getNumElements();
-
   unsigned SelfInitializedElt = TheMemory.getNumElements();
   unsigned SuperInitElt = TheMemory.getNumElements() - 1;
 
-  // We might need an extra bit to check if self was consumed.
-  if (HasConditionalSelfInitialized)
-    ++NumMemoryElements;
-
   // Utilities.
 
   auto destroyMemoryElement = [&](SILLocation Loc, unsigned Elt) -> SILValue {